1. Field of the Invention
The present invention relates to an optical object measurement apparatus, and in particular relates to an optical object measurement apparatus wherein a predetermined area of an object to be measured is irradiated with a light beam from a light source, and optical interference is used to detect light reflected from the object in order to derive therefrom tomographic information of the object.
2. Description of the Prior Art
Several apparatuses have been known for using a laser light source or another light source to observe a shape of a three-dimensional surface of an object or tomographic information of an interior of a sample. For example, it is known that a confocal optical microscope can improve resolution in the depth direction better than a normal optical microscope, and allow the shape and structure of a surface of a microscopic sample surface to be observed and measured.
Document D1, Japanese Laid-open PCT Publication No. 1989-503493, discloses an optical object measurement apparatus as an example of one technique in a confocal optical system, wherein a sample is irradiated with a light beam emitted from a laser light source via a rotating disk having a pinhole array, and light reflected from the sample again passes through the rotating disk and is detected.
Document D2, Japanese Laid-open Patent Publication No. 1994-242380 (U.S. Pat. No. 3,294,246), discloses a configuration in which a sample is scanned using a light beam from a laser light source passing through a pinhole disk, and interference light that is formed from light reflected from the sample and reference light from a reference mirror is detected. It is suggested in Document 2 that adjusting a distance between the sample to be measured and an objective lens allows a three-dimensional shape of a surface of the sample to be accurately measured from a position at which an interference signal reaches maximum light intensity.
Meanwhile, optical coherence topography (OCT) apparatuses involving the use of interference from a low-coherent light (partially coherent light) have been receiving attention over the past several years. The OCT makes it possible to examine and observe predetermined tomographic images of an object to be measured in a contactless and noninvasive manner, and are starting to be applied as scanning devices in medicinal, biological, and industrial fields.
For example, Document D3, Japanese Laid-open Patent Publication No. 1992-174345 (Japanese Examined Patent Application (Kokoku) No. 1994-35946), discloses an example of early OCT wherein means is provided for generating a frequency-shifted reference light beam from an irradiation light beam, and a light beam reflected from an object to be measured is combined with the reference light beam to produce a beat component, which is detected to provide an tomographic image of the object.
Document D4, Japanese Laid-open PCT publication No. 1994-511312 (U.S. Pat. No. 3,479,069), discloses a configuration of OCT comprising a light source having short coherence length characteristics, an interferometer having an optical fiber, phase modulation means, a transverse-direction scanning mechanism, an ultrasonic light modulator, means for controlling an optical path length, and the like. Document D4 discloses a basic technique wherein interference light guided via an optical fiber is detected and processed to efficiently obtain a tomographic image of the sample with the aid of a confocal effect due to the optical fiber.
Document D5, Japanese Laid-open Patent Publication No. 2000-126188 (U.S. Pat. No. 3,318,295), discloses an optical tomography observation apparatus including a light source for generating low-coherent light and an optical fiber interferometer. The apparatus is usefully combined with a structure constituting an end part of an endoscope, body cavity scope, or other instrument via one optical path of the interferometer. Document D5 discloses a technique involving the use of the endoscope or other instrument inserted into a body cavity, wherein a CCD or other device provided as a conventional observation device is used to provide a two-dimensional reflected image of a diseased tissue and a tomographic image in the depth direction thereof that is obtained by detecting an interference signal from an interferometer.
Document D6, Japanese Laid-open Patent Publication No. 1998-332329 (U.S. Pat. No. 3,332,802), discloses a configuration that employs a semiconductor laser light source with a beam frequency capable of being swept, a Michelson interferometer, and a one- or two-dimensional image-capturing device. A video signal that is output in the frequency sweeping period is subjected to Fourier transformation to produce a tomographic image. Such a system is advantageous in that a sweeping mechanism for performing a mechanical movement in the optical axis direction is not additionally required, a stable interference optical system can be established, and measurements can be obtained in a short amount of time.
Document D7, Japanese Laid-open Patent Publication No. 1999-325849, discloses a configuration wherein a light beam is split into a reference arm and a measuring arm, and a spectrograph is used to detect the intensity of light that is produced by interference between a measuring beam obtained via the measuring arm and a reference beam obtained through the reference arm. The reference arm is provided with means for changing the phase of the light, and a signal from the spectrograph is analyzed to produce an optical tomographic image of a transparent, partly transparent, or opaque object.
Document D8, Japanese Laid-open Patent Publication No. 2001-330558 (U.S. Pat. No. 3,594,875), discloses a system wherein a light beam from a light source is split into a signal optical path that passes through an object to be measured and a reference optical path that travels via a predetermined reflecting mirror, and an interference optical system is provided with two CCD sensors for receiving a periodically isolated interference beam that is split into two. The two CCD sensors receive interference light pulses that have different phases and produce signals, which are processed to provide image information of an interior layer of the object.
Document D9, Japanese Laid-open Patent Publication No. 2005-241464, discloses a system wherein a light beam from a light source is split into a signal optical path that passes through an object to be measured and a reference optical path that travels via a predetermined reflecting mirror, and an interference optical system is provided with three CCD sensors for receiving an interference beam that is split into three. The three CCD sensors receive interference light pulses that have different phases and produce signals, which are processed to provide image information of an interior layer of the object.
Document D10, Japanese Laid-open Patent Publication No. 2005-245740, discloses an OCT apparatus with an interferometer comprising means for forming a plurality of interference images differing in phase, and means for extracting the plurality of interference images by high-speed switching. The plurality of extracted interference images are detected using CCD image sensors, and the plurality of detected images is processed to form a tomographic image.
However, an interferometer is not used in Document D1. Therefore, the apparatus in Document D1 cannot be used to observe the interior of a sample, as is done with OCT or another form of tomography. By contrast, an interferometer is used in Document D2. However, a low-coherence property of the light source is not used. Additionally, background light associated with scattering in the interior of the sample is not removed. For these and other reasons, the apparatus cannot be used for tomographic observation or measurements of the interior of a sample such as with OCT.
Documents D3 through D5 disclose basic OCT systems (“time-domain method”), in which a reflecting mirror associated with a reference beam is controlled to move in the optical axis direction to scan the object in the depth direction thereof. Therefore, the focus of the irradiation light directed on the object to be observed cannot be optimally maintained for the entire region of the tomographic image, and complications are encountered in achieving higher levels of resolution in the in-plane direction orthogonal to the optical axis (depth direction).
Document D6 discloses an OCT system called “swept source method” in which a light source with a light wavelength swept is used. Problems are presented with this system in that a specialized type of laser light source that can stably control the frequency of the light over a desired range is required; a light source of this type is limited in terms of variety, wavelength range, and other aspects; and the light source itself is expensive.
Document D7 discloses an OCT system called “spectral-domain method” in which a detector in the form of a spectrograph is used in the detection system. Advantages of this system are that the tomographic information is extracted based on numeric calculations, making mechanical scanning in the depth direction unnecessary; however, problems are presented in that the characteristics of the spectrograph limit the measurement range in the depth direction, and resolution is not readily improved in the direction orthogonal to the depth direction.
Documents D8 through D10 disclose novel OCT systems in which a two-dimensional image-capturing device is used in the detector. Problems are presented with this system in that when an image is captured of a living organism or other object having a strong scatter factor, the presence of strong background light superimposed as a direct-current component on the detector causes saturation in the image-capturing device, and makes it difficult to improve the gradation of a signal component that contains tomographic information. Problems are also presented in that it is difficult to maintain precise alignment between the two CCDs in Document D8, and the three CCDs in Document 9. Furthermore, drawbacks are presented in Document D10 in that the light source is not used efficiently, and a high-cost, specialized switching light source or other such configuration is necessary when the system is implemented in practice.
It is therefore an object of the invention to provide a highly practical, simpler and less expensive optical object measurement apparatus capable of observing a tomographic image of an object with high levels of resolution and contrast and capable of being applied for measurement thereof.